The present invention relates to a Raman amplifier, for example for use in an optical fibre communications system.
A Raman amplifier is a well known amplifier configuration. This amplifier uses conventional fiber (rather than doped fibers), which may be co- or counter-pumped to provide amplification over a wavelength range which is a function of the pump wavelength. The Raman amplifier relies upon forward or backward stimulated Raman scattering. Typically, the pump source is selected to have a wavelength of around 100 nm below the wavelength over which amplification is required.
Raman amplifiers are increasingly being used to improve the overall gain characteristics of high capacity optical wavelength division multiplexed (WDM) communications systems. Raman amplifiers have the advantage that they do not attenuate signals outside the wavelength range over which amplification takes place. However, high power pump sources are required, and it may be difficult in practice to implement pump sources of the required pump wavelength and power. In addition, it is usual to provide a separate pump sources for each wavelength required, typically in the form of separate Raman fibre lasers or semiconductor pumps.
Raman amplifiers can be designed as discrete (off-the-shelf) components, in which case the fibers used and the pump sources will be selected to achieve specific gain requirements. Typically, a number of pump sources are provided in order to provide a flat gain spectrum. There will always, however, be wasted pump power, as not all pump power is used within the amplifier. It is known that output can be provided from the discrete amplifier to which unused pump light is routed.
Raman amplification can also be induced in the fiber used for transmission of the optical data signal. This arrangement is known as a distributed Raman amplifier. In a distributed Raman amplifier, the maximum pump power is limited by eye safety issues, as there is a risk in distributed systems of an operator being exposed to the optical pump signal in the event of a fiber cut or other disconnection within the system. For example, a maximum pump power of around 500 mW may be appropriate.
It would be desirable to increase the efficiency of discrete Raman amplifiers by improving the pump power usage.
According to the invention, there is provided an optical amplifier comprising at least first and second stages in which the second stage comprises at least one pump source and has an output for unused pump power, and wherein the first stage comprises a distributed Raman amplifier pumped using the unused pump power from the second stage.
This arrangement enables unused pump power to be used in a distributed Raman amplifier, in particular using the transmission fiber at the input to the second stage amplifier. The second stage preferably comprises a discrete Raman amplifier.
The second stage is preferably counter-pumped, and the unused pump power propagates into the transmission fiber at the input of the second stage, the transmission fiber at the input of the second stage thereby forming the first stage of the amplifier.
A filter may be provided at the input of the second stage for shaping the intensity profile of the unused pump power propagating from the second stage to the first stage. The unused residual pump power will be dependent on the characteristics of the second stage amplifier. Typically, the shorter wavelength pumps will be attenuated more than the longer wavelength pumps, and the filter is used to reshape the residual pump spectrum before using it in the distributed first stage amplifier.
Alternatively (or additionally) one or more additional pump sources may be provided for providing additional pump power to the first stage. This enables the gain of the combined amplifier to be equalised through control of the additional pump sources. In particular, the second stage may be counter pumped using a plurality of pump sources of different wavelengths, and wherein the one or more additional pump sources are for providing, in combination with the unused pump power, a desired intensity profile of the pump power provided to the fist stage.
The additional pump sources do need to be high power sources, as they are only required to alter the pump power spectrum sufficiently to flatten the overall gain spectrum. This enables gain equalization without significant losses resulting from filtering operations.
The invention also provides a wavelength division multiplex (WDM) optical communications system comprising a transmitter for generating signal radiation of wavelength in an operating wavelength range, a receiver for receiving for detecting the signal radiation, and an optical fiber link between the transmitter and the receiver, wherein one or more optical amplifiers are provided in the link, at least one amplifier comprising an amplifier having at least first and second stages in which the second stage comprises at least one pump source and has an output for unused pump power, and wherein the first stage comprises a distributed Raman amplifier pumped using the unused pump power from the second stage.
The invention also provides a method of amplifying optical signals, the method comprising supplying optical signals for amplification to the first stage of a multiple stage Raman amplifier, the multiple stage Raman amplifier comprising at least first and second stages, the first stage comprising a distributed Raman amplifier, the method comprising amplifying the signal in the first stage by providing a pump signal to the first stage, the pump signal including unused pump power from the second stage, and subsequently amplifying the signal in the second stage.
The pump signal to the first stage may include additional pump power selected such that the combined pump powers provide a desired pump power spectrum.